This invention relates to techniques for measuring nonlinearity in the operation of transmission systems and, in particular, to an arrangement and a method of precisely measuring selected intermodulation distortion products produced by noise loading of individual components, such as repeaters designed for high capacity, wide bandwidth transmission systems or any portion of such systems.
A conventional technique of testing an entire channel of a transmission system for nonlinear operation is by measuring the intermodulation distortion in the output signal of the channel introduced by the nonlinearity of the channel. This technique is generally known as noise loading and entails a source of random noise signals which are initially lowpass filtered. The filtered noise signal has statistical properties similar to an actual message load of several independent signals and is used to simulate same. The output of the lowpass filter is then band elimination filtered somewhere within the pass-band of the channel to provide at least one quiet band or notch in the filtered noise signal. When the latter signal is applied to the channel, nonlinear operation of the channel produces intermodulation distortion products some of which appear in the frequency band of the notch. Generally, these distortion products are dominantly second and third order products and to a much lesser extent higher orders of intermodulation distortion. Those distortion products in the frequency band of the notch, typically both second and third order distortion products, are selected from the signal output of the channel by a passband filter designed to pass signals at selected frequencies in the band of the notch. Some form of detector is used to measure the level of the distortion products and indicate the amount of non-linearity of the channel under test.
Although the conventional noise loading approach is usually adequate for high intensity or readily detectable intermodulation distortion products such as encountered when evaluating overall channel performance, this technique is unsuitable for low intensity intermodulation distortion products at levels that are substantially less than ambient thermal noise levels. Low intensity intermodulation distortion products, which contribute to the overall distortion of the channel, are generally the contribution of segments or portions of the channel, for example, active components such as repeaters.
In order to increase the strength of the distortion products selected by filtering relative to the ambient noise, a process called signal averaging is used which requires a noise that is periodic. Successive sampling of the periodic signal at the same phase in the cycle enables detection of intermodulation products at levels substantially less than the ambient thermal noise level. Since the thermal noise is not periodic its contribution to the detected signal diminishes with successive samples while the intermodulation distortion products remain constant. Typically, a pseudorandom noise generator, which is a shift register with feedback, produces a random like signal that is periodic. This signal is lowpass filtered to provide a normal or Gaussian amplitude distribution of spectral components. The Gaussian distribution is desirable to produce a noise loading signal that closely resembles an actual message signal load transmitted by the system to simulate operating conditions. Substantial conformity to a Gaussian distribution requires the cutoff frequency of the lowpass filter to be at least an order of magnitude below the clock frequency which operates the shift register in the pseudorandom noise source. Because of the foregoing relationship, present shift registers do not operate fast enough to provide a Gaussian type noise signal which simulates a message load for a wideband transmission channel (e.g., bandwidths of 1 to 150 MHZ or more).
Another disadvantage is that the conventional technique is not able to resolve the third order intermodulation distortion from the usually much more intense second order distortion produced when a wideband noise loading signal is subjected to minor nonlinearities of individual transmission components. Throughout a transmission system, third order distortion is coherent and therefore cumulatively increases as the number of serial repeaters employed in the system increases, while second order distortion remains more or less constant. Thus, separate evaluation of the third order distortion produced in the presence of second order distortion by each individual repeater is essential to predict overall performance of a transmission system. Such a system may be a submarine cable system or a single-sideband cable radio transmission system. Accurate evaluation of component nonlinearity also requires a testing arrangement with a sensitivity about 50 times greater than such a test on an overall system.
An object of this invention is to obtain accurate measurements of intermodulation distortion products of low intensity typically produced by individual system components of transmission systems.
A related object is to provide a periodic pseudorandom noise signal having a high density spectrum over a broad bandwidth that emulates Gaussian distributions in both the frequency domain and time domain.
A related object is to separate different orders of intermodulation distortion products so their individual intensity can be measured which would otherwise coincide in frequency due to the high spectral density and broad bandwidth properties of noise loading signals required to simulate broadband transmission signals.